1. Field of the Invention
The present invention relates to a susceptor support shaft for epitaxial growth apparatuses and an epitaxial growth apparatus. The present invention relates in particular to improvement of a susceptor support shaft for supporting a susceptor on which a semiconductor wafer is placed during growth of an epitaxial film on the semiconductor wafer.
2. Description of the Related Art
In the field of semiconductor electronics in which products are increasingly required to achieve high performances and to have high functionality, quality of an epitaxial wafer significantly influences the quality of a resulting product device. An epitaxial wafer is formed by growing an epitaxial film on a surface of a semiconductor wafer by vapor phase epitaxy. The epitaxial film formed is of high quality in that it has crystal axes aligned in accordance with atoms regularly oriented in the surface of the semiconductor wafer.
For production of such epitaxial wafers, batch-process-type epitaxial growth apparatuses capable of simultaneously performing epitaxial growth on a plurality of semiconductor wafers have been conventionally used. However, such batch-process-type epitaxial growth apparatuses as described above are unsuitable for production of large-diameter semiconductor wafers. For this reason, in recent years, it has been common to use single-wafer epitaxial growth apparatuses in which individual semiconductor wafers are separately subjected to epitaxial growth. Recently, there have been developed single-wafer epitaxial growth apparatuses for large-diameter semiconductor wafers, capable of processing semiconductor wafers having a diameter of 450 mm or more.
FIG. 1 is a schematic sectional view of a conventional single-wafer epitaxial growth apparatus 200. This epitaxial growth apparatus 200 includes a chamber 201, a susceptor 202 for supporting a wafer W placed inside the chamber 201, and a susceptor support shaft 203 for supporting the susceptor 202. A supply port 204 for process gases is formed at a side portion of the chamber 201, and an exhaust port 205 is formed at a position of the chamber opposite to the supply port 204. Further, a plurality of halogen lamps 206 as heating sources are radially disposed in each of an upper region and a lower region of the chamber 201.
FIG. 2 is a schematic exploded perspective view showing details of the susceptor 202 and the susceptor support shaft 203. The susceptor support shaft 203 includes a support column 207 and three arms 208, and each of the arms 208 is provided with a support pin 210 for supporting the susceptor 202. Further, recessed portions 211 are separately formed on the rear surface of the susceptor 202 at positions corresponding to the respective support pins 210. The recessed portions 211 are engaged with the support pins 210 of the arms 208, so that the susceptor 202 is positioned with respect to the susceptor support shaft 203. As described above, the conventional susceptor support shaft 203 generally supports the susceptor 202 at three points using the three support pins 210 provided on the three arms (See JP 2000-124141A).
In the epitaxial growth apparatus 200, the semiconductor wafer W is placed on the susceptor 202; the halogen lamps 206 are lit to heat the semiconductor wafer W; and at the same time, a carrier gas, a growth source gas, a dopant gas, and the like are introduced as process gases from the supply port 204, such that the process gases flow in a laminar flow state along a surface of the semiconductor wafer W which has been heated to a predetermined temperature while an exhaust gas is discharged from the exhaust port 205. At this point, in order to uniformly form an epitaxial film on the entire surface of the semiconductor wafer W, the susceptor support shaft 203 is rotated around the support column 207 as a central axis to rotate the susceptor 202 and the semiconductor wafer W. Thus, an epitaxial film can be grown on the semiconductor wafer W by the epitaxial growth apparatus 200.
However, when an epitaxial film is formed by the above conventional method, in-plane resistivity of the epitaxial film greatly varies. In response to this, the inventors of the present invention earnestly studied to make resistivity distribution of an epitaxial film uniform, and consequently found that, even in a case where the susceptor 202 is uniformly heated, the peripheral portion of the susceptor 202 is deflected (in the circumferential direction) due to exposure of the susceptor 202 itself to the high-temperature environment and that a magnitude of such deflection of the susceptor 202 as described is especially large at portions not supported by the susceptor support shaft 203.
In a case where the epitaxial growth process is carried out in a state where the susceptor 202 has partially been deflected at the peripheral portion thereof, a space is created between the deflected portion of the susceptor 202 and a semiconductor wafer W placed thereon. A carrier gas or the like entering into the space partially cools the semiconductor wafer W and the temperature of the semiconductor wafer W varies in the circumferential direction thereof. When epitaxial growth is performed on such a surface of the semiconductor wafer W where the temperature varies as described above, the amount of dopant taken into the epitaxial layer grown on the wafer surface would also vary in the wafer plane. In short, deflection of the susceptor 202 significantly affects the resistivity distribution of the silicon epitaxial film in the epitaxial wafer. This problem of non-uniform resistivity distribution cannot be ignored in an epitaxial wafer to which the resistivity standards must be strictly applied.
In view of the above, the inventors thought of and studied performing of epitaxial growth in a state where in-plane temperature variation of a semiconductor wafer due to deflection of a susceptor is suppressed by increasing the numbers of arms and support pins to support a susceptor at more points. Consequently, they found that resistivity distribution of an epitaxial film could be made uniform by increasing the numbers of arms and support pins to support a susceptor at more points. However, they found another problem in that increased number of arms would make the arms interrupt between a surface on the rear side of a susceptor and a pyrometer, which would make it impossible to detect accurate temperature of the surface on the rear side of the susceptor using the pyrometer. When accuracy in detecting the temperature of the surface on the rear side of the susceptor is reduced, the power of lower halogen lamps 206 cannot be controlled accurately, which would result in reduced quality of the epitaxial wafer. Further, it is found that accuracy in detecting the temperature of the surface on the rear side of the susceptor by the pyrometer tends to be reduced as the rotation speed of the susceptor is increased. To produce a flatter epitaxial wafer, the susceptor and the semiconductor wafer are required to be rotated at a higher speed. Therefore, these problems would be more significant as more attempts to produce wafers having higher flatness are made in the future.
On the other hand, if the number of arms is reduced to the original number so as not to impair the accuracy in detecting the temperature of the rear surface of a susceptor by a pyrometer, in-plane resistance variation of an epitaxial film due to deflection of the susceptor cannot be suppressed. Thus, there have been trade-offs.